QStringConverter: Add missing include on Windows

It's relied on implicitly, which is no longer valid in dev.
And may accidentally be broken in other branches.

Pick-to: 6.3 6.2
Change-Id: I2272b6914e883e20d0989a1762eb1a5c1aef4e0e
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/text/qstringconverter.cpp

@@ -48,6 +48,9 @@
 
 #ifdef Q_OS_WIN
 #include <qt_windows.h>
+#ifndef QT_BOOTSTRAPPED
+#include <QtCore/qvarlengtharray.h>
+#endif // !QT_BOOTSTRAPPED
 #endif
 
 #if __has_include(<bit>) && __cplusplus > 201703L